The present invention relates to a method of manufacturing semiconductor devices, and more specifically, to a method of forming wiring layers.
Aluminum (Al) and aluminium alloy (Al alloy) are widely used as material for wiring layers of semiconductor devices because of their low electric resistivity and high workability. resistivity and high workability. On the other hand, however, there exists a problem in that electromigration or stress migration easily occurs in wiring layers formed of Al or Al alloy. To prevent the above-mentioned defectiveness, although copper (Cu), titanium (Ti), lead (Pb) and/or silicon (Si) is added to the Al alloy, the effect of preventing the above-mentioned defectiveness is not yet so far satisfactory, because of the advance of wiring micronization.
In particular, electromigration and stress migration easily occur at interfaces between crystal grain boundaries. Therefore, the conventional countermeasures are such that an Al wiring layer is divided into several sublayers and Al oxide films (Al.sub.2 O.sub.3) are interposed between the two divided Al wiring sublayers, respectively in order to prevent the interfaces from being arranged in the film thickness direction.
FIG. 16 shows a cross-sectional structure of conventional wiring layer. In the drawing, a silicon oxide film 72 is formed on the surface of a semiconductor substrate 71, and a first Al wiring sublayer 73 is formed by depositing Al or Al alloy on the oxide film 72 by sputtering. Further, another Al oxide film 76 is formed on the surface of the Al wiring sublayer 73 by oxidization. Here, there are several oxidization treatment methods; that is, the semiconductor substrate 71 formed with the Al wiring layer 73 is left within the atmosphere; the substrate is naturally oxidized within an oxide atmosphere; the heated substrate is left within the atmosphere or an oxide atmosphere; the substrate is kept in a pure water, etc.
In the above-mentioned methods, however, since the substrate is oxidized on the basis of natural oxidization, it is difficult to control the film thickness of the Al oxide film 76 with excellent reproducibility. In the case of an Al alloy including an additive such as Si in particular, local battery cells are formed therein and therefore, the film thickness of the Al oxide film tends to increase, so that the effect of separating the crystal grain interfaces is markedly subjected to the influence of the thickness of the Al oxide film. Namely, in the conventional manufacturing method, there exists a problem in that it is impossible to form an Al wiring layer excellent in electromigration resistance, stress migration resistance, and film thickness reproducibility, so that the reliability of the Al wiring layer is not high.
In the conventional manufacturing method, additionally, there exists another problem in that the productivity is low. FIG. 17 shows the conventional manufacturing process. In the drawing, the semiconductor substrate 71 formed with the silicon oxide film 72 is carried into an Al film forming equipment (in step 81); the Al film forming equipment is evacuated into a vacuum (in step 82); the first Al wiring sublayer 73 is formed on the surface of the silicon oxide film 72 on the semiconductor substrate 71 by sputtering (in step 83); the Al film forming equipment is deevacuated into the atmospheric pressure (in step 84); and the Al oxide film 76 is formed on the Al wiring sublayer 73 by means of either one of the above-mentioned oxidization methods (in step 85). The above steps 82 to 85 are repeated to form the Al wiring sublayer 74 and the Al oxide film 77, and the Al wiring sublayer 75 in sequence. In the conventional manufacturing method as described above, since the Al film forming equipment is repeatedly evacuated or deevacuated whenever an Al oxide film is formed, the productivity is low and therefore the manufacturing cost is high.
Further, contact holes or via-holes are easily formed on the surface of the Al wiring layer. To eliminate these holes, the semiconductor substrate is heated up to 460.degree. C. or higher to melt Al or Al alloy when the Al wiring layer is being formed by sputtering. Further, since the melted Al or Al alloy is poor in wetting with respect to the silicon oxide film, a thin base film of titanium (Ti) or titanium nitride (TIN) must be once formed on the silicon oxide film, and thereafter the Al wiring layer is formed on the base film by sputtering at high temperature.
In the conventional method, however, there exists another problem in that a void is easily produced within a hole in the high temperature sputtering process. With reference to FIG. 18, in more detail, a hole 95 is formed in a silicon oxide film 91 on a semiconductor substrate 90, and a Ti thin film 92 is formed on the surface of the oxide film 91. In this case, a void 94 is produced at the hole 95 of the Al wiring layer 93. The existence of the void 94 deteriorates the resistance against electromigration or stress migration, thus causing lower reliability. The above-mentioned voids 94 increase with increasing microminiaturization of holes.
In summary, in the conventional method of manufacturing semiconductor devices, there exist various problems in that it is difficult to form Al wiring layer excellent in electromigration resistance, stress migration resistance, and reproducibility, so that the reliability and productivity of the Al wiring layer are not high.